Safety valves are frequently used downhole to provide emergency closure to prevent blowouts. A common design incorporates a pressurized control system involving a control line running from the surface to the valve body downhole. Pressure on the control line moves a piston connected to a hollow tube inside the valve body, known as a flow tube. A circular seat is provided for the valve member, which has a conforming shape. The valve member, known as a flapper is mounted for 90-degree rotation. When the flow tube is advanced downhole, it contacts the flapper and pushes it off the seat. The flapper winds up behind the flow tube in the wide-open position. To close the valve, pressure is removed from the control line or other control action is taken so that the flow tube is forced in an uphole direction. Eventually the flow tube clears the flapper and a spring on the flapper urges it to rotate 90 degrees until it comes to rest in sealing contact with the conforming valve seat.
Prior designs have used a variety of torsion springs to urge the flapper to the closed position. One prior design is illustrated in FIG. 1 of U.S. Pat. No. 6,328,062. Here the torsion spring is mounted around a pin extending through a pair of parallel ears sticking out from the flapper. The torsion spring has one end fixed and the other end has a tab that contacts the flapper. When the flapper is pushed open by the flow tube, the torsion spring winds up to store a closure force to act on the flapper when the flow tube is later retracted. Other types of flappers have a single broad ear that extends from an edge and the torsion springs are connected at opposed ends of a mounting pin that rotates with the flapper. The rotation of the flapper winds up the torsion springs. This layout is shown as FIG. 2 of U.S. Pat. No. 6,328,062. The problems with both such designs were explained in detail in this patent and the invention in that patent tried to address the design weaknesses of the prior art. The problem was end failure of the torsion springs particularly in the hook or tab area that contacted the flapper or its mounting pin. The proposed solutions were to use a universal joint connection at the connection interface to reduce fatigue failures at this location or to use an alignment rod running through the curved torsion spring disposed outside the periphery of the flapper. The problem with these solutions was the difficulty in manufacturing parts to have the required strength in the limited space available and the added cost of the parts and the labor to assemble them in place.
Other flapper closure designs are illustrated in U.S. Pat. Nos.: 4,019,532; 4,407,325; 4,531,587; 5,156,374; 6,003,605; 5,411,056 and GB Applications 1,308,954; 1,563,487; 2,198,170 and 2,236,549 and PCT Application WO86/05853.
What is needed and what is one of the desired objectives of the present invention is to be able to provide a simple and inexpensive closure mechanism that can be reliable in operation over the many years a safety valve is in operation downhole. The preferred solution involving a rack and pinion accomplishes this objective. Other variations are also contemplated and the full scope of the invention will be appreciated by one skilled in the art from a review of the detailed description of the preferred embodiment and the claims, which appear below.